The air permeable sheet material in the art is provided to have a micro porous structure by forming a hydrophobic film comprising the phase separated layer containing the filler such as BaSO4 and CaCO3 and heterogeneous polymer, by micro-dispersing the easily extractable component and extracting the component, or by forming micro continuous foamed structure.
Such films having micro porous structure has been used for the backsheet of hygiene products such as disposable diaper and sanitary napkin, or has been in heavy usage for waterproof material for rainwear and sports wear by laminating it to nonwoven fabric, woven fabric knitted fabric.
In addition, the sheet member having a selective permeability is widely used for the hygiene products such as the absorbent product including diaper, and mask shutting off the microscopic particles such as dust, microorganism and pollen. Such a member includes a material supporting activated carbon in the powder form, activated carbon fiber and zeolite.
Generally, these sheet members require a profile in dependent on its application. Such a profile includes, for example in the absorbent products, air permeability and water resistance along with retaining water absorbability in dependence on the constituent element of the products. In addition, such a profile, in the hygiene products such as mask, includes to have a profile inhibiting the passage of the above-mentioned microscopic particle, along with air permeability. In order to obtain such a profile, the securement of water resistance and impermeability can be attained to reduce the pore size of the sheet member. However, the reduction of the pore side leads to the limitation of the air permeability thereby hard to obtain the necessary profile for the applications.
There is the so-called breathable backsheet as the sheet member having air permeability. Many proposals for manufacturing this backsheet have been made including the layered products of microfiber web and the method utilizing the gel blocking. However, one and only effective mean as commercially adapted is to use the air permeable PE film as the backsheet. However, the ventilation rate of the normal air permeable film is in the range of about 3.0 to 5.0 kg/24 hr·m2 of MVTR (Moisture Vapor Transfer Ratio), in accordance with ASTM E96-63F or E96-80B. Accordingly, it has to say to provide not enough profile on the practical use.
That is, according to several researched for the stuffy of underwear and the others, 2 mm or more of gap between the underwear surface and the skin surface will be necessary for generating the surface airflow due to the bellows effect to inhibit to being stuffy, along with high air permeability. In case of the normal underwear, 1.0 sec/100 mL of air permeability of the underwear expressed as Gurley method (JIS P8117) is possessed. In comparison with the air permeable film, the normal underwear has 1 kg/min·m2 or more of ventilation rate expressed in accordance with ASTM D-737, the thin-type underwear has 10 kg/min·m2 or more of ventilation rate, and the wetted underwear has at least 0.5 kg/min·m2 or more of moisture vapor transfer ratio. These ventilation rates correspond to about 300 times of a ventilation rate of the above-mentioned conventional air permeable film. Therefore, the conventional air permeable film is not enough for inhibiting the generation of being stuffy and rash.
The present inventors considers that a necessary air permeability for an absorbent product such as diaper is at least 100 sec/100 mL of air permeability as expressed in accordance with the Gurley method, preferably tens seconds/100 mL or lower of air permeability. In addition, there is little gap for providing the bellows effect in the current absorbent product such as diaper, since the absorbent product is defined to closely contact with the body as possible without any gaps. Accordingly, there is little surface airflow of the product. Furthermore, a small air permeability of the air permeable film is no help in the product after the body fluid has adsorbed in the absorbent, since the absorbent comprises itself a thick layer of pulp/SAP. It is naturally that, in case of wearing the absorbent product being in such state, the humidity and temperature within the product will be increased and the product will be stuffy and rash will be occurred.
In addition, there are several proposals that any ventilation holes are provided in the side, front, rear and end of the product although a risk of occurring any leakages from the product is recognized. However, there is occurred a new problem that excretions and smells originated from the excretions are leaked through the ventilation hole.
That is, in order to be capable of manufacturing an absorbent product which has highly air permeability and that there is less chance to occur stuffy and rash, it is necessary to improve air permeability of the backsheet, and further to provide stable air permeable structure in the absorbent.
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